1. Field of the Invention
The present invention relates to intensifying phosphor screens for use in radiographic imaging, and particularly intensifying screens with antistatic agents therein.
2. Background of the Art
There are at least two critical objectives in the production of radiographic images, particularly in medical radiographic images. One desired aspect of radiographic imaging is, of course, the faithfulness of the generated image compared to the object through which radiation was passed during imaging. Another important aspect, particularly during medical radiographic imaging, is the reduction of the level of exposure of the object (patient) to radiation during the imaging process.
One significant means of reducing the level of X-ray exposure during imaging has been the use of "intensifying screens" during the imaging process. These screens usually comprise phosphors in a binder on a carrier layer. The phosphors absorb X-ray radiation at a higher efficiency than does silver halide which is normally used in the hard-copy output of radiographic images. The phosphors not only absorb X-rays at an efficient rate, but can also phosphoresce (or fluoresce), emitting radiation at a wavelength other than the wavelength of X-rays which the phosphor absorbed. Depending upon the chemical nature and properties of the phosphor, the emitted radiation may be at essentially any wavelength between and including the infrared and ultraviolet wavelengths of the electromagnetic spectrum. Silver halide naturally absorbs radiation in the ultraviolet and near blue wavelengths, and can be spectrally sensitized to efficiently absorb radiation in other portions of the ultraviolet, visible and the infrared regions of the electromagnetic spectrum. By exposing the phosphor screen to X-rays, having the phosphor screen emit in the UV, visible or infrared, and having a silver halide emulsion spectrally sensitized to the wavelength of emission of the phosphor screen and optically associated with the phosphor screen, the entire efficiency of the X-ray imaging system can be greatly enhanced. This allows for the use of lower doses of X-rays during exposure of the object.
The use of such phosphors is well known in the art as exemplified by such patents as U.S. Pat. Nos. 3,883,747 and 4,204,125 where there is direct emission of phosphorescent radiation upon X-ray stimulation, and U.S. Pat. Nos. 3,859,527 and 5,164,224 where there is exposure to X-rays, storage of the absorbed energy by the phosphor, and subsequent stimulation by stimulating radiation to cause the phosphor to emit the stored energy as UV to infrared radiation. These phosphor systems are commercially successful and provide a significant benefit to the radiographic art. In these types of systems, however, there is a trade-off between speed and sharpness. To absorb more x-rays and emit more light, the screen itself can be made thicker. But in this case, light generated within the thickness of the screen is scattered by the phosphor grains to a greater extent, thereby reducing the resulting image sharpness recorded on the film. Conversely, to improve sharpness a thinner screen is desirable, but this reduces the x-ray absorbing power, and ultimately requires a higher dosage to the patient or object being x-rayed.
Many methods of improving the image quality, particularly the sharpness of images generated from phosphor screens, without adversely affecting the sensitivity or speed of the system, have been proposed. Reflective particulates, dyes, pigments and other light affecting materials have been proposed as additives to phosphor layers to improve sharpness as shown in EPO 102 790 (powdered glass), Japanese Application 146,447/1980 (white pigments), Japanese Patent Application 163,500/1980 (colorants), and EPO 175 578 (sputtering or vacuum evaporation of phosphors).
The objective of these methods is primarily to provide a high concentration of phosphor in the active layer of the screen and provide a screen of uniform properties. U.S. Pat. No. 5,306,367 produces a storage phosphor screen by dispersing phosphor particles in a thermoplastic binder diluted with a solvent, then coats the mixture, dries to remove the solvent, and compresses the coating at a temperature above the melting point of the binder. U.S. Pat. No. 5,296,117 deposits phosphor particles in a binder by electrophoretic deposition of a dispersion of the phosphor particles in a solution of polymeric binder. The solution is coated onto a substrate, dried and the phosphor screen thus produced. Each of these types of systems has shown some benefits, but there is still significant room for improvement in the sharpness of radiographic phosphor screens. In particular, it is desired to eliminate complicated deposition processes which can be costly, to eliminate the use of solvents which are harmful to the environment, and to eliminate or reduce high processing temperatures.
Another problem with photographic imaging systems is static charges interfering with image quality or the ability to move the photographic media. The contact of the photographic medium with other surfaces during transportation or placement of the medium can cause static or tribolelectric charges to be created on the medium. These charges not only interfere with the physical movement of the film by causing attraction to surfaces which prevent slippage between the film and those surfaces, but the charges can also discharge as sparks which can create spurious images on the photographic medium.
Numerous antistatic layers and antistatic agents have been added to photographic media, especially graphic arts and radiographic photographic imaging media (industrial and medical photographic elements) to reduce the various types of static charging which can occur with these imaging materials. Although many of these antistatic layers, systems and additives can reduce various types of static buildup, each new system may have different physical requirements and need different antistatic protection. It is usually desirable for antistatic layers to be able to provide surface resistivity levels of less than 10.sup.13 ohms/square, more preferably less than 5.times.10.sup.12 ohms/square, and more preferably less than 10.sup.12 ohms/square to assure better antistatic protection.
U.S. Pat. No. 4,666,774 discloses the use of antistatic agents in the oxidatively treated protective topcoat of a radiographic intensifying screen. Many different classes of antistatic agents are used in that topcoat layer.
U.S. Pat. No. 4,845,369 discloses a radiation image storage panel wherein at least one layer contains fibrous conductive material to reduce static buildup.
U.S. Pat. No. 5,151,604 discloses radiation storage phosophor panels which contain electroconductive zinc oxide whiskers in at least one layer to reduce static charging in the panel.
European Patent Application 0 377 470 A1 discloses radiation image storage phosphor panels in which at least one layer contains an antistatic agent comprising an inorganic salt of a metal.
Japanese Patent Publication 91-200731/27, JP94034120-B2 discloses the use of a fluorinated surfactant in a surface layer on a lead, absorbing X-ray screen to reduce static.
The need for antistatic protection in radiation image storage panels and intensifying screens has been recognized in the art. It would be particularly desireable to accomplish this antistatic protection without providing an additional coating to the surface of the screen, such as when the antistatic agents are included in a protective topcoat. It also would be desirable to produce screens or panels which provide antistatic protection by incorporation of antistatic agents which are resident in the surface of such screens. The antistatic topcoat is described as preferably comprising a lacquer applied to the surface of the element.